1. Field of the Invention
This invention relates generally to optical communication systems, and more particularly to an improved structure and apparatus for a low-cost, high-performance, free space or fiber optic interconnect and data link.
2. Discussion of the Prior Art
Optical systems are presently being used for high bandwidth, high-speed voice and video communications. As a result, optical systems are one of the fastest growing constituents in the communications systems market. The expression “optical system,” as used herein, relates to any system that uses an optical signal to transport data and or application content across an optical medium. Previously, most optical systems were configured as single channel systems carrying a single wavelength over an optical medium such as a fiber optic cable or some form of free space interconnect. As the demand for broadband services grows, the increase in traffic has led to a need for greater channel carrying capacity. Due to the high cost of expanding the transport facilities of an optical communications or network system, increasing the capacity by laying more cable, for example, is generally impractical. Thus, it has become important to develop a technique that could expand the channel-carrying capacity of these existing facilities.
One technique for increasing the capacity of an optical communications or network system is to multiplex the data streams or signals from a number of sources. In general, multiplexing divides the available bandwidth of a common transport medium into fractional units that can be accessed by individual sources. With time division multiplexing (TDM), for example, these fractional units are referred to as time slots. TDM is a scheme by which the contents from multiple sources are divided into a plurality of segments. Each segment is then inserted into a pre-assigned time slot and transmitted via a composite carrier signal in a predictable, specified order via a single optical channel. At the other end, the data streams are then reconstructed and routed to the appropriate receiver. While this is a useful technique for conveying data or content from a plurality of sources on a single channel, the dispersion and non-linearity of the optical media tend to limit the performance and transmission capacity of the channel.
A practical method to increase the total capacity of a transport medium beyond the capacity of a single channel is to create a number of signals or channels having different non-interfering wavelengths. These signals or channels are then transmitted through a single optical transport medium such as fiber optic cable or, in the case of some very short-haul optical communications systems, a free space interconnect by using a technique called wavelength division multiplexing.
Wavelength division multiplexing (WDM) converts the optical signals from several sources into a set of carrier channels, each having a specified, non-interfering wavelength, that can be simultaneously transmitted over a single optical fiber or free space interconnect. Since each channel is completely isolated from the others, these discrete channels are simply combined or mixed, thereby creating an array of “virtual optical channels” that can be transported through a single optical transport medium.
FIG. 1 is a block diagram illustrating a typical WDM-based optical communications system as implemented in the prior art. In a typical WDM optical system, wavelength multiplexing and demultiplexing components are used to combine and separate the individual wavelengths, respectively. As FIG. 1 shows, a plurality of transmitters 110 direct a plurality of signals, each having a different wavelength (λ1, λ2, λ3, . . . λn) from an array of data sources 112 to multiplexer 102 that combines these signals into a single multiplexed signal 120. The multiplexed signal is then transmitted through a fiber optic cable 106. At the receiving end, demultiplexer 104 receives and separates the combined signal into individual signals according to wavelength (λ1, λ2, λ3, . . . λn) and then directs these signals to the appropriate receivers 108. For a signal to be properly demultiplexed, a single optical channel is selected from the multiplexed optical signal. The demutilplexer is designed to separate the individual wavelengths based on the precise center wavelength and bandwidth for each channel. One method of demutiplexing is to use an optical splitter followed by optical filters to select the precise bandwidth of each channel.
The advantage of WDM is that it significantly expands the capacity of a fiber optic communications facility. For example, a four-channel WDM optical system will experience as much as a four-fold increase in capacity and performance as compared to a conventional single channel system. The problem, however, is that such an arrangement requires a precise correspondence between the wavelength of each optical signal and the wavelength selection element for each channel in order to avoid “crosstalk,” that is, interference between adjacent channels. Also, most of the conventional multiplexing and demultiplexing components used in such systems or networks tend to suffer from performance deficiencies requiring some form of amplification to be added to the configuration. As a result, these systems are very complex and have a significant impact on the cost of increasing the capacity of an existing optical communications or network system.
FIG. 2 is a schematic diagram that illustrates a prior art scheme for a WDM optical data link. One approach used to reduce the cost of a WDM optical data link is an array of optical signals that are transmitted by a monolithic, very tightly coupled VCSEL-based transmitter array 202 to a channel-matched, wavelength-selective, monolithic photodetector array 204 through a single multimode fiber optic cable 206 without the use of either a waveguide combiner or distributor. The transmitter array and the detector array are mounted to respective substrates 212 and 216 by suitable conductive means such as solder balls 210. This is accomplished by circularly arranging all the elements of VCSEL-based transmitter array to be in line with the core of fiber optic cable 206 so that the entire VCSEL-based transmitter array is simultaneously interfaced to the fiber optic cable. An adhesive bond 214 such as epoxy resin is used to couple both VCSEL-based transmitter array 202 and photodetector 204 array to either end of fiber optic cable 206. It should be noted that the VCSEL-based transmitter array and the photodetector array are designed in a conventional cylindrical shape. Additionally, transmitter array 202 and photodetector array 204 have flat planar-surface structures that facilitate proper alignment with fiber optic cable 206. The disadvantage of this technique is that if either the VCSEL-based transmitter array or the photodetector array is not precisely aligned with the fiber optic cable or does not exactly match the outer circumference of the fiber optic cable, the optical data link will experience a significant level of loss, thus reducing the overall performance of the optical link. Also, the VCSEL-based monolithic transmitter array and monolithic photodetector array must have wavelengths that match. Otherwise significant crosstalk and interference may occur at the receiver.
Similarly, in a short haul, free space communications environment, wavelength dependent multiplexing and demultiplexing components are used to direct the signals from a transmitter array through an open space to a photodetector array at the other end. As before, an array of transmitter elements, each emitting an optical signal having a different wavelength, is directed to a combiner or multiplexer. The combiner merges the received signals into an aggregate signal and transmits it across the specified open space or channel to a corresponding demultiplexer. At the receiving end, the optical signals are split and directed through separate optical paths, and the separated optical signals are then detected by a photodetector that is tuned for a particular wavelength.
Like the fiber optic based communications facilities described above, a short haul, free space facility that employs an optical multiplexer and demultiplexer will also experience a performance deficiency. In this case, it can be as much as a 12 dB loss per channel for a four-channel WDM link. In addition, such a configuration requires precise alignment between the transmitter and the wavelength selection element of each detector in order to avoid interference between adjacent channels. Another disadvantage is that if the transmission elements of a transmitter array are too tightly grouped together, it will also generate additional signal interference and electrical crosstalk due to interference from the driving signals.
In view of the foregoing, there exists a need for a two-dimensional, free space optical data link or interconnect that employs an array of vertical cavity surface emitting lasers (VCSELs) to transmit multiple optical signals simultaneously to an array of detectors that selectively receive the transmitted optical signals. Ideally, such an optical interconnect is more cost effective, provides greater capacity and higher performance than other conventional optical interconnects.